The damselfly Platycypha caligata has males with abdomens coloured blue dorsally, and laterally expanded tibiae coloured white anteriorly and red posteriorly. The females are cryptically brown coloured. The males are strongly territorial and centre their territories around potential oviposition sites (driftwood or treeroots in the water). Territorial interactions involve a complex, hierarchical series of flights which include flash displays of the white and red surfaces of the tibiae. Most successful mating interactions followed a central sequence: (1) a male intercepts a female passing through his territory, (2) by displaying his blue abdomen behind him he attracts her to his oviposition site, (3) she lands and makes probing oviposition movements on it, apparently testing its suitability for oviposition (possibly on the basis of softness), while he courts her by displaying his white tibiae while hovering around her, (4) if she accepts the site she flies up slowly, perches, and they copulate, whereafter he returns her in tandem to the site to oviposit. Females apparently rejecting the site decamped rapidly. Sometimes males courting already ovipositing females were successful. All other interactions, especially those away from oviposition sites were unsuccessful (only 104 of 564 observed interactions led to copulation). This mating behaviour is compared with that of other Odonata, and especially Calopteryx maculata. It is speculated that the evolution of this female choice of oviposition site prior to mating and the elaborate male courtship resulted from male territorial defence of the only available suitable oviposition sites. The adaptation of Platycypha caligata to oviposition on driftwood and treeroots (the discreet, scarce, defendable sites invoked as the first step in this theory) may have been associated with their occupation of mountain streams which have no emergent vegetation.